Pantheon Mechanics Question

Pantheon Mechanics Question

Basically I am wondering why all the guides seem to insist on Arpen Marks and items for Pantheon, yet his recomended 1st starting item is for mana regeneration and 2/4 of his abilities scale with AD, the other half AP.

Does his spear shot(Q) and heartseeker strike(E) do extra damage if you have Arpen? I was under the impression they were spells with physical dmg only, as neither applies on hit effects like Gangplanks Parry does - do they count as spells or hits? Wouldnt it be better to go for All-out AD with some base CoolDownReduction to get the most benefit from his spells early on? Also if u stack loads of AD, your normal hits do loads of dmg anyway right, so why bother with Arpen?

Isnt Arpen best for auto-attack type DPS heroes, like Ashe etc? yes, he is melee AD, but Pantheon is a caster at heart, not a "hard-carry" right-click win type, only his passives aegis protection and certain death require that he actually physically hit stuff. .

Also why the heck are his W and R APbased and is there a viablie build that incorporates some AP?

If anyone could clear it up for me I really would appreciate it, my head hurts when I try to do the math.